Conventional photolithography is believed to be limited to about 150 nm in pattern dimensions. While X-ray and ion beam lithography have been demonstrated as viable alternative techniques for creating pattern dimensions below this limit, they are expensive. E-beam lithography has also been proven as a viable technique. However, it is time consuming and, like X-ray and ion beam lithography, expensive. In contrast to such lithographic techniques, imprinting offers an attractive alternative to the fabrication of two-dimensional (2-D) nanometer-scale features, as a result of simpler, faster, and cheaper processing, making this technique a potential replacement for photolithography in mass production.
The above-mentioned lithographic techniques are further limited to fabrication of 2-D and supported features. Imprinting, however, can lend itself to the fabrication of three-dimensional (3-D) features, wherein 3-D features comprise structural variation with depth. Three-dimensional patterning techniques are likely to be important enabling technologies for a number of applications. In microelectronics, for example, the third dimension could possibly allow the speed and memory of microprocessors to go beyond the limitations currently imposed by 2-D features. In optoelectronic industries, 3-D photonic band gap structures are garnering considerable attention because 3-D structures serve to minimize loss of light [Kiriakidis et al., “Fabrication of 2-D and 3-D Photonic Band-Gap Crystal in the GHz and THz Region,” Mater. Phys. Mech., 1:20-26, 2000]. In drug/chemical delivery systems, sensing systems and catalysis, the feasibility of fabricating 3-D structures will provide breakthroughs in the efficiency of controlled delivery, sensing, and selectivity in chemical reactions. For example, a sphere with a meshed surface can be envisioned as a chambered pill that contains multiple drugs or a multifunctional catalysis support.
While 2-D fabrication techniques are mature technology down to the sub-micrometer scale, very little has been reported regarding 3-D sub-micrometer fabrication techniques. Currently, of the limited amount of literature available on sub-micrometer 3-D fabrication techniques, most reports are seen to be mere extensions of various photolithography techniques. For instance, Whitesides et al. have shown that a porous microsphere can be obtained via a self-assembly approach [Huck et al., “Three-Dimensional Mesoscale Self-Assembly,” J. Am. Chem. Soc., 129:8267-8268, 1998], and Yamamoto et al. have demonstrated the fabrication of micrometer scale grooved structures using deep X-ray lithography [Tabata et al., “3D Fabrication by Moving Mask Deep X-ray Lithography with Multiple Stages,” The Fifteenth IEEE International Conference on Micro Electro Mechanical Systems, 180-183, 2002]. Whitesides et al. have also reported on a “membrane folding” method used to create 3-D structures [Brittain et al., “Microorigami: Fabrication of Small Three-Dimensional Metallic Structures,” J. Phys. Chem. B, 105:347-350, 2001]. While most of these techniques have demonstrated the feasibility of creating 3-D sub-micrometer or nanometer scale features, they are not easily implemented for mass production.
Both conventional nano-imprinting [Sun et al., “Multilayer resist methods for nanoimprint lithography on nonflat surfaces,” J. Vac. Sci. Technol. B,]6(6):3922-3925, 1998] and reversal imprinting [Huang et al., “Reversal imprinting by transferring polymer from mold to substrate,” J. Vac. Sci. Technol. B, 20(6):2872-2876, 2002] techniques are attractive alternatives to the above-mentioned techniques in the fabrication of 3-D nano-features, although currently both techniques create 3-D structures through multiple imprinting on patterned substrates or on substrates with topology. A more efficient imprinting technique would, therefore, go a long way in solidifying imprinting's role as a potential replacement for currently used lithographic patterning techniques.